Classic Corned Beef and Cabbage

Name: Classic Corned Beef and Cabbage
Ingredients: 1 flat-cut corned beef brisket (about 4 pounds ); 1/4 teaspoon mustard seed; 1/4 teaspoon coriander seed; 1/8 teaspoon whole cloves; 1/8 teaspoon whole peppercorns; 2 to 3 bay leaves; 1 large onion, peeled and quartered; 6 Yukon Gold potatoes (about 1 pound ), halved, or quartered, if large; 1 small head cabbage (about 1 3/4 pounds), outer tough leaves removed and cut into 6 wedges; 12 slender carrots (about 1 pound ), peeled and left whole; 2 teaspoons kosher salt; 2 tablespoons chopped fresh parsley , for garnish; Dijon or grainy mustard, for serving; 1 cup sour cream; 3 tablespoons whole milk; 1 tablespoon lemon juice, or more to taste; 1 tablespoon prepared horseradish in brine, or more to taste; 1/2 teaspoon Dijon mustard; 1/4 teaspoon salt; 1/8 teaspoon ground black pepper; 1/4 cup chopped fresh parsley